1. Field of the Invention:
The present invention relates generally to free electron lasers and, more specifically, to a high gain, single pass free electron laser (FEL) with a new pulse power undulator capable of producing extremely high magnetic fields with a very short period.
2. Description of the Related Art
Ordinary gas and solid-state lasers are monochromatic; i.e., they can generate only specific wavelengths corresponding to energy transitions in their lasing media. Dye lasers can be tuned over a narrow range but require a gas laser for optical pumping and can operate only at relatively low power levels.
In contrast, the free electron laser as described in Madey U.S. Pat. No. 3,822,410 presents an extremely adaptable source of coherent radiation because it can be tuned to virtually any wavelength, and it operates at high power. In a free electron laser, high-energy electrons (i.e. electrons that have been accelerated to velocities approaching the speed of light) travel in a beam through a vacuum instead of remaining attached to the atoms of a lasing medium. Because the electrons are free, the wavelength of the radiation they emit is not confined to a particular wavelength corresponding to a permitted transition between two energy levels of an atom. Radiation is produced when the high energy electron beam is passed through a transverse, spatially periodic magnetic field produced by an assembly of magnets known as an undulator. The magnetic field of the undulator bends the beam of electrons back and forth in the traverse direction. Each time an electron in the beam is deflected, it emits a burst of synchrotron (broadband or incoherent) radiation. If the laser is appropriately designed so that the oscillations add to each other, the combination of individual bursts yields a beam of coherent radiation at a wavelength approximately given by: ##EQU1## where:
.lambda..sub.r is the wavelength of coherent light (in cm)
.lambda..sub.o is the undulator period, i.e., the distance between adjacent magnets of opposite polarity (in cm).
.gamma. is the electron energy divided its rest mass energy; and
k is a parameter defined by ##EQU2## where B is the rms of the magnetic field (in Tesla).
As seen from the equation (1) above, the output wavelength of a free electron laser can be tuned by varying the electron energy (proportional to .gamma.). For short wavelength lasing (in the x-ray region of the spectrum), a free electron laser needs electron energies on the order of 1 GeV, which is extremely high.
The size of the FEL is also a problem because the distance between adjacent poles of the magnet is limited by equation (2). Since k .varies. B.lambda..sub.o and it is desirable for k.apprxeq.1, if .lambda..sub.o (the distance between adjacent N and S poles of a magnet) is decreased by a factor of 10, B must be increased by a factor of 10, and 10 Tesla is impractical for ordinary magnets, requiring on the order of 100 poles in series.
In addition to the difficulty of obtaining the necessary magnetic field strength and electron beam emittance, the gain of the device becomes a problem for emission in the short (&lt;1000 Angstroms) region of the spectrum. If the single-pass gain is less than unity, it becomes necessary to pass the photon beam axially back and forth through the undulator to obtain lasing (light amplification). In ordinary free-electron lasers, this is accomplished by mirrors at opposite ends of the undulator structure. However, there are no mirrors capable of reflecting x-rays. The solution is to make the laser superradiant, resulting in light amplification in a single pass through the undulator. See, e.g., R. Bonifacio and F. Casagrande, "The Superradiant Regime of a Free Electron Laser", Nuclear Instruments and Methods in Physics Research A239 (1985) pp. 36-42.
To obtain superradiance, the electron beam must be very dense (i.e., it must have an extremely high brilliance and must be of low emittance), and the magnetic field strength must be very strong with a very short period.